Recently, advances in technology for monitoring a physical condition in real-time have been made. To this end, a bio-signal can be used for checking a physical condition in real-time. A bio-signal measurement can be achieved by an interface for handling bio-signals and a technique for monitoring the bio-signals. For example, the interface for handling the bio-signals may generate a command or an instruction for detecting bio-signals or operating a medical device in response to the bio-signals. The technique for monitoring the bio-signals allows for recognizing distinct features of the bio-signals in order to, for example, determine bodily diseases or diagnose a mental illness.
There are various measurable bio-signals such as an Electroencephalography (EEG), an Electrocardiography (ECG), an Electromyography (EMG), a Photoplethysmography (PPG), an Electrooculography (EOG), breathing, and the like. Among them, the PPG is one of the more easily monitored bio-signals in real-time.
A PPG signal, which is detected based on optical characteristics of a measuring device, shows a heart rate synchronized with cardiac impulses. A pressure generated by cardiac impulses makes blood flow through blood vessels. The generated pressure may affect the bloodstream at capillaries in fingers, toes, earlobes, or the like. Arterial blood flowing through capillaries in the distal of body can be supplied into cellular tissues, while returning into the heart via veins. Accordingly, whenever cardiac impulses occurs, a blood flow rate of the arterial blood flowing through capillaries in the distal of body is changed, i.e., increased or decreased.
Meanwhile, when a light source of a PPG measuring sensor emits a light into a part of body, some of the light is absorbed into blood, bone, and/or tissue while the other of the light is penetrated or reflected so as to enter into a light receiving device. Absorbed light subtracted from an original emitted light can equal a penetrated light or a reflected light recognized by the light receiving device. The change of the penetrated or reflected light may show a blood flow change. Thus, the PPG measuring sensor uses the light receiving device to detect an amount of light so that the blood flow change synchronized with the cardiac impulses can be measured.
An apparatus for measuring the PPG signal may be implemented relatively simply and can readily recognize a user's movement. However, since the PPG signal can be distorted by noises caused by the user's movement, the apparatus should remove recognized noises while handling the PPG signal. In order to effectively process the noises, a wrist-worn device can be used.